Boiling water nuclear reactors have reactor cores composed of two moderator flow regions. These regions include the flow region through the reactor core and the flow region through the so-called core bypass region. In understanding these flow regions, the construction of the regions will first be set forth. Thereafter, the thermal hydraulic and nuclear characteristics of both regions will be discussed.
The reactor core includes a plurality of side-by-side fuel bundles, these bundles being square in section and vertically elongate in dimension. The fuel bundles each include a matrix of sealed and vertically upstanding fuel rods supported on a lower tie plate. The lower tie plate serves to admit water moderator through the bottom of the fuel bundle about the rods for steam generation. A upper tie plate typically fastens to at least some of the fuel rods and permits the exit of water and generated steam from the fuel bundle. A channel surrounds the lower tie plate, the upper tie plate and the fuel rods therebetween. This channel serves to confine the flow path through the fuel bundle. At the same time, this channel and its flow path separate the fuel bundle flow interior of the channel from the core by-pass region exterior of the channel.
So-called fuel rod spacers are placed at selected vertical intervals along the length of the fuel bundle. These spacers maintain the otherwise long a flexible fuel rods from coming into abrading contact one with another under the dynamics of fluid flow within the reactor as well as to maintain the designed fuel rod to fuel rod spacing for optimum nuclear performance. This being the case, each fuel rod spacer defines a matrix position for each fuel rod at the particular elevation of the spacer. Each fuel rod is confined by the spacer to a designed spaced apart position with respect to all of the adjacent fuel rods.
The spacers are typically surrounded at a band. It is the function of this band to provide an outer defining envelope for the spacer cells into which fuel rods can be placed.
Assembly of the fuel bundle can be summarized. Typically the lower tie plate and spacers are placed in their final spatial relation. Thereafter the fuel rods are threaded through the spacers at each matrix position and registered to the lower tie plate. Thereafter the upper tie plate is fitted over the assembly. Finally, the fuel bundle has the channel placed over its exterior surface.
The operation of the fuel bundle from the thermal hydraulic stand point can be simply stated. Water moderator coolant is inlet from the bottom of the fuel bundle through the lower tie plate. Increasing fractions of vapor are generated as the moderator passes upwardly within the channel between the fuel rods through the fuel bundle with exit of the water and generated steam at and through the upper tie plate at the top of the fuel bundle.
Operation of the fuel bundle from the nuclear stand point can likewise be simply stated. The water moderator within and about the fuel bundle takes the fast neutrons generated by the atomic reaction and slows or thermalizes these neutrons when the neutrons pass through the moderator. In the slow or thermalized state, the neutrons are capable of promoting the continuous chain reaction required to keep the reactor operating. It is to be understood that the density of the water moderator is an important factor in allowing the nuclear reaction to continue. Where the moderator is relatively dense--as for example where it consists of pure water--the fast neutrons are rapidly thermalized and the reaction abundantly continues. Where the moderator is not dense and contains large vapor fractions--the fast neutrons are not rapidly thermalized and among other things the reaction is not as abundant in its continuation.
Having summarized the operation of the fuel bundles, the construction of the core bypass region can now be set forth. Simply stated, the core bypass region is defined by the exterior of the fuel bundles as they are arrayed within the reactor core in spaced apart side-by-side relation. As the square sectioned fuel bundles are placed together, they are spaced apart so as to define cruciform sectioned (that is "cross shaped") interstitial spaces. These spaces interconnect in a continuous matrix between all the fuel bundles. This interconnected matrix defines the core bypass region. This region accommodates the reactor control blades during reactor shut down and is flooded with water during reactor operation.
The function of the reactor control blades is well known. These control blades are typically cruciform sectioned members. These cruciform sectioned members typically fit interstitially in the complimentary shaped cruciform core bypass region defined by the adjacent but spaced apart fuel bundles. When the cruciform shaped control blades are inserted to the cruciform shaped interstices between the fuel bundles of the core bypass region, they control and even shut down the nuclear reaction. The control blades in a boiling water reactor are typically inserted from below the reactor between the fuel rods displacing water in the core bypass region and absorbing the thermal neutrons.
The function of the core bypass region during operation is also well known. The control rods are in large measure withdrawn. Water occupies this region immediately upon control rod withdrawal and immediately adjoins the fuel bundles at the channel walls. The water is on the outside of the channel walls; the fuel is on the inside of the channel walls. The water in this core bypass region--when not displaced by the fuel rods--serves further to moderate the fast neutrons emitted by the nuclear reaction to the slow or thermalized state where these neutrons may continue the nuclear reaction. In this sense, the core bypass region is a particularly important source of nuclear moderator immediate the outside of every fuel bundle.
Having explained the nuclear function of the core bypass region, attention can be directed to the fuel rods within the fuel bundle immediately adjacent the channel. This attention will first consider the unique nuclear position of these fuel rods and thereafter the thermal hydraulic limitations of these fuel rods.
From a nuclear operational view, the fuel rods adjacent the channel are typically the most reactive fuel locations in the fuel bundle of a boiling water reactor. Because the moderator of the core bypass region is immediately available, these fuel rods--especially in the early life of a fuel bundle--tend to be the most reactive. Consequently, they generate the most power and easily come under so-called "critical power" limits. When a fuel rod approaches critical power limits, the heat generated by the fuel bundle exceeds the ability of the coolant to remove the heat; the excess heat become a threat to the integrity of the cladding of the fuel rod surrounding the nuclear fuel. When this limit is approached, the entire remainder of the fuel bundle is limited in performance so that the critical power limit is not exceeded at any one individual portion of a fuel bundle.
From the thermal hydraulic operational view, the peripheral fuel rods must be provided with an adequate flow of coolant to prevent these fuel rods from exceeding the critical power limits.